ABSTRACT

The distributions of trace elements in ferromanganese concretions (FMCs) and the surrounding terra rossa (TR) matrix overlying dolomite have been examined to extend our understanding of their mobilization, redistribution and fractionation during karst weathering and pedogenesis. This study demonstrates that the characteristics of trace elements in TR have been severely overprinted by weathering and pedogenetic processes. The composition of trace elements in FMCs within TR soil profiles varies greatly with depth. The concentrations of Be, Ni, Cu, Zn, As and Bi in FMCs fluctuate and tend to increase with depth; other trace element concentrations fluctuate in the profile but show no clear trend. The toxic elements Cr, As, Cd, Tl and Pb, as well as Co, are significantly enriched in FMCs relative to the surrounding TR matrix: arsenic exhibits strong affinity with Fe, but Cr and Cd contents are not significantly correlated with either Mn or Fe. Correspondingly, Co, Tl and Pb are related to the Mn in FMCs rather than the Fe. In FMCs, trace element distribution and partitioning in the Mn-Fe phases are governed by the location-specific formational pedoenvironment, with the weathering front etching downwards. Experiments with simulated acid rain indicate that desorption of elements from FMCs is generally pH-dependent regardless of the variation of the released amounts and their desorbability. Moreover, it seems that toxic elements bonded to Mn-phase were easily desorbed from FMCs. As a result, the bioavailability, mobility and activity of some toxic elements in FMCs and TR are potentially increased in field areas experiencing severe soil erosion and acid rain.